The availability of techniques for culturing preimplantation mouse embryos has provided the means for microsurgically producing uniparental embryos which have developed to term and possess a homozygous-diploid number of chromosomes from only one parent. These gynogenetic embryos are genetically analogous to mouse parthenote embryos with the exception that the egg was activated by fertilization. Spontaneously activated parthenote embryos from the LT inbred strain of mice shortly after implantation and LT uniparental embryos are being produced to determine the developmental competence of the homozygous LT maternal genome for elucidating the cause of parthenote deaths. Other studies are concerned with the nongenetic contributions of sperm necessary for viable development.